{"id":229,"date":"2024-05-21T18:09:00","date_gmt":"2024-05-21T18:09:00","guid":{"rendered":"https:\/\/permutationcity.co.uk\/bp\/?p=229"},"modified":"2024-07-27T08:52:20","modified_gmt":"2024-07-27T08:52:20","slug":"functional-programming-in-c","status":"publish","type":"post","link":"https:\/\/permutationcity.co.uk\/bp\/2024\/05\/21\/functional-programming-in-c\/","title":{"rendered":"Functional Programming in C++"},"content":{"rendered":"\n

I remember functional programming<\/a>\n being a niche aspect of computing science.\nSoftware engineering didn’t consider it at all.\nNow you can find discussion and books about using it in any major programming language.\nI read Functional Programming in C#<\/a> but, to me,\nthis oversold the technique while it lacking practical advice for day to day programming.\nI had a much better time with\nFunctional Programming in C++<\/a>\nwhich concentrates a lot more on how to get things done.\nIt tells you about a new tool and how to add it to your toolbox.<\/p>\n\n\n\n

Functional programming<\/h2>\n\n\n\n

Unsurprisingly this paradigm is all about functions, specifically\npure functions<\/em><\/a>.\nThese are functions where their output is entirely dependant on their input and they have no side-effects.\nThat means they don’t depend on any global state and they don’t change any global state.\nThis makes them easier to reason about, less likely to cause or be effected by bugs,\neasier to test and debug, well-suited to parallelisation.\nI tend to think of functional techniques as force multipliers,\nthey let you get more for less.<\/p>\n\n\n\n

As well as using functions this paradigm creates and manipulates functions.\nSo you will be passing functions into functions and \/ or getting functions back as a result.\nIn C++ this means you’ll be getting into template meta-programming.\nThis is a powerful technique but is also confusing.\nYou have to learn how to use a bunch of build time equivalents to runtime code.\nExpect build time error messages to be less helpful than you’re use to.\nOn the other hand this book gives some good examples to learn from and reason to do so.<\/p>\n\n\n\n

Procedural to functional<\/h2>\n\n\n\n

The book starts with a good example of taking some simple code using the procedural paradigm and\nconverting it to a functional one.\nLet’s take a list of filenames and turn it into a list of line counts.\nI’ll paraphrase it here:<\/p>\n\n\n\n

std::vector<uint32_t> CountLinesInFiles(const std::vector<std::string>& filenames) {\n    std::vector<uint32_t> lineCounts;\n    char character = 0;\n    for (const auto& filename : filenames) {\n        uint32_t lineCount = 0;\n        std::ifstream stream(filename);\n        while (stream.get(character)) {\n            if (character == '\\n') {\n                lineCount++;\n            }\n        }\n        lineCounts.push_back(lineCount);\n    }\n    return lineCounts;\n}<\/pre><\/div>\n\n\n\n
This code does the job although I’d have probably broken it into two functions from the start.<\/p>\n\n\n\n
uint32_t CountLinesInFile(const std::string& filename) {\n    std::ifstream stream(filename);\n    return std::count(std::istreambuf_iterator<char>(stream), std::istreambuf_iterator<char>(), '\\n');\n}\n\nstd::vector<uint32_t> CountLinesInFiles(const std::vector<std::string>& filenames) {\n    std::vector<uint32_t> lineCounts;\n    for (const auto& filename : filenames) {\n        lineCounts.push_back(CountLinesInFile(filename));\n    }\n    return lineCounts;\n}<\/pre><\/div>\n\n\n\n
That what the book does next.\nIt’s also making use standard library to do some of the work.\nDespite the extra function it’s taking up less space.<\/p>\n\n\n\n
std::vector<uint32_t> CountLinesInFiles(const std::vector<std::string>& filenames) {\n    std::vector<uint32_t> lineCounts(filenames.size());\n    std::transform(filenames.cbegin(), filenames.cend(), lineCounts.begin(), CountLinesInFile);\n    return lineCounts;\n}<\/pre><\/div>\n\n\n\n
More standard library work to transform a vector of one things into a vector of another things.<\/p>\n\n\n\n
std::vector<uint32_t> CountLinesInFiles(const std::vector<std::string>& filenames) {\n    return filenames | transform(CountLinesInFile);\n}<\/pre><\/div>\n\n\n\n
The book introduces the pipe operator to take a collection and push it through<\/em> a transform.\nThe book came out before C++20 which includes std::ranges<\/code> and\nthe pipe operator so there may be some discrepancies.<\/p>\n\n\n\n
std::vector<uint32_t> CountLinesInFiles(const std::vector<std::string>& filenames) {\n    return filenames | transform(OpenFile) | transform(CountLines);\n}<\/pre><\/div>\n\n\n\n
Finally we break up CountLinesInFile<\/code> to two function concepts OpenFile<\/code> and CountLines<\/code>,\nnot included, both of which could be used in other contexts.<\/p>\n\n\n\n
All of this sounds good to me and has gone towards my options on\nrepeating code<\/a>.\nWhether you can take all<\/em> your code and do something like this seems less important to me.\nI think there is definitely some<\/em> code that could be improved like this.<\/p>\n\n\n\n
Functional in C++<\/h2>\n\n\n\n
After this the book the books takes you through some functional programming fundamentals:<\/p>\n\n\n\n